server : speculative checkpointing (#19493)

server : speculative checkpointing (#19493)

* server : speculative decoding using checkpoints

* server : fix draft check with checkpoints

* server : rename spec vars

* server : log levels

* server : refactored spec logic to speculative.cpp

* server : renamed spec checkpoints option

* server : fix spec checkpoints, logging

* speculative : checkpoints with draft model, logging

* server : n_tokens_cur and create_checkpoint in draft

* server : fix server_speculative_callback (slot.id)

* spec : fix ngram-map/begin idx_last_check

* spec : init ckpt (begin() wasn't called)

* chore: update webui build output

* server : restore sampler in spec checkpoint and clear mem

* cont : avoid --spec-use-checkpoints argument

* cont : remove server_prompt_checkpoint_with_size

* spec : rename (leave_draft_state)

* cont : clean-up

* cont : do not ignore partial drafts even if the are short

* cont : spec callback owned by session

* cont : simplify

* cont : avoid empty speculative session

* cont : simplify

* cont : simplify

* cont : enable mtmd speculative decoding

* cont : keep the spec sampler alive

* cont : simplify

* cont : fix nullptr deref + draft checkpoints

* cont : remove common_speculative_accept_response

* cont : remove callback

* cont : simplify

* cont : minor

* cont : simplify

* cont : fix accepted number

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Co-authored-by: Georgi Gerganov <redacted>